1. Field of the Invention
The present invention relates to an optical information recording medium, such as DVD (Digital Versatile Disc) or CD (Compact Disc), in which a large amount of information such as letters, sound and animation is recorded and/or recordable as digital signals, a process for the preparation thereof, and further a photo-curable transfer sheet advantageously utilized in the medium and process.
2. Description of the Related Art
As an optical information recording medium in which digital signals are in advance recorded by forming pits on its surface, CD and CD-ROM are widely used. Recently, DVD that the animation can be also recorded by forming pits on its both (double) sides has been noted as the next generation recording medium instead of CD and increasingly used. Further, attention is directed to recordable discs such as CD-R, DVD-R and DVD-RW having groove(s) or grooves and pits thereon.
Conventional DVD having recording layers (surfaces) on its both sides includes a disc readable from double sides, as shown in FIG. 9, in which each of reflective layers 1a, 2a is formed on a signal-pits surface of each of transparent resin substrates 1, 2 having the signal-pits surface on its one side, and the two transparent resin substrates 1, 2 are bonded to each other through an adhesive layer 3 such that the reflective layers 1a, 2a are faced to each other; and a disc readable from single side, as shown in FIG. 10, in which a semitransparent reflective layer 1b is formed on a signal-pits surface of a transparent resin substrate 1 while a reflective layer 2a is formed on a signal-pits surface of another transparent resin substrate 2 and the two transparent resin substrates 1, 2 are bonded to each other through an adhesive layer 3 such that the semitransparent reflective layers 1b and the reflective layer 2a are faced to each other.
The DVD readable from double sides can be prepared, for example, by subjecting melted polycarbonate resin to injection molding by the use of a stamper having unevenness (concave and convex) corresponding to the reverse of unevenness of the signal-pits to be recorded on the substrate to prepare a transparent resin substrate having unevenness on its surface, forming a reflective layer on the uneven surface by spattering metal such as aluminum on it, and bonding two transparent resin substrates obtained in the above manner to each other through an adhesive such that the two reflective layers are faced to each other.
For example, a standardized specification of a next generation optical disc “Blu-Ray Disc” was proposed on Feb. 10, 2002. The specification mainly includes storage capacity of 23.3/25/27GB, laser wavelength of 405 nm (violaceous laser), lens numerical aperture (N/A) of 0.85, disc diameter of 120 mm, disc thickness of 1.2 mm and track pitch of 0.32 μm.
In the Blu-Ray Disc, as mentioned above, the sizes of groove and pits are reduced, and therefore it is required to reduce a spot size of a reading laser. The reduction of the spot size results in that the reading is apt to be influenced by inclination of the disc, and hence even DVD having a little warpage cannot be reproduced (read out). In order to avoid the disadvantage, it may be effective to reduce a thickness of a substrate and make a thickness of a cover layer provided on pits surface on a laser-irradiation side to approx. 0.1 mm.
“NIKKEI ELECTRONICS”, Nov. 5, 2001, pp.68 describes a process for the preparation of DVD suited to the above-mentioned requirements. The process is explained by referring to FIG. 11. A UV curable resin 5A is provided, by application, on a reflective layer (or recording layer) 6a of a disc substrate (1.1 mm) 4a having the reflective layer on its uneven surface, while a UV curable resin 5B is provided, by application, on a stamper 4b made of polycarbonate having uneven surface. Subsequently, after the substrate is turned over, the turned-over substrate and the stamper are put together, and the UV curable resins 5A, 5B are cured by irradiating ultraviolet beam from the stamper side. The stamper 4b is then removed from the cured UV curable resin 5B, a reflective layer (or recording layer) 6b is formed on the uneven surface and further a cover layer (thickness of approx. 0.1 mm) 7 is provided the reflective layer 6b. 
In the process described in NIKKEI ELECTRONICS, a UV curable resin (layer) is provided on a disc substrate and a stamper by application, and the substrate is turned over and bonded to the stumper. Hence, the process is needed to perform complicated procedures such as the application and turning over steps. In more detail, when the turned-over substrate and the stamper are put together through viscous UV curable resins, bubbles are generated in the vicinity of an interface of the combined UV curable resins. Hence it is difficult to successfully bond the UV curable resins of the substrate and stamper to each other. Furthermore, the UV curable resin is accompanied by large shrinkage on its curing, and hence the resultant medium is apt to have deformation such as warpage.
A process for the preparation of an optical information recording medium such as DVD capable of solving the above disadvantages is disclosed in WO 03/032305 A1. In the process, used is a photo-curable transfer sheet having a photo-curable transfer layer comprising a photo-curable composition, the photo-curable composition being deformable by application of pressure and containing a reactive polymer having a photopolymerizable functional group. In more detail, the photo-curable transfer sheet in the form of solid instead of the above UV curable resin is used when depression of the stamper, and therefore it is possible to transfer the uneven surface into the sheet to avoid the above disadvantages.